The Gran Chaco or Chaco Boreal is a large lowland plain that today is part of Brazil, Bolivia, Paraguay and Argentina. 

From the late nineteenth century, there had been simmering tension between Paraguay and Bolivia as to the extent of their control of the area as control of the area was of key strategic interest to both countries. Bolivia, in particular, saw the access to the Atlantic via the Paraguay River as important, especially as they had lost access to the Pacific in a war with Chile 1879. 

Oil companies had been present in Bolivia and speculation arose that the Chaco region was rich in petroleum resources. 

The lure of the land and the potential riches from the oil was enough to spark a war between the two countries over the disputed region. 

Despite having superior numbers, Bolivian forces struggled against the guerilla tactics of the Paraguayans, who also had tactical support from Italian and French forces. The Chaco War ended with a ceasefire agreement on June 12th 1935.